The present invention relates, in general, to a tensioner, and more particularly to a chain tensioner as used in chain drives of internal combustion engines for tensioning the chain during operation.
German Pat. No. DE 40 35 823 C1 describes a hydraulic tensioner for traction drives, in particular chain drives in internal combustion engines. A tensioner piston is pressed against the chain and received in a cylinder for longitudinal displacement. The piston and the cylinder together define a pressure chamber for hydraulic fluid. As the piston moves in the direction of the chain to tension it, the pressure chamber expands, thereby creating a negative pressure. As a consequence, a check valve opens and motor oil is able to flow into the pressure chamber. A tensioning of the chain moves the piston inwards away from the chain, so that the pressure chamber is reduced in size. During inward movement of the piston, motor oil migrates from the pressure chamber through a leakage gap. This conventional chain tensioner is further provided with a pressure restrictor valve which opens, when the pressure in the pressure chamber is excessive. Thus, the characteristic damping curve of the chain tensioner becomes softer, once the pressure restrictor valve opens.
The configuration of the leakage gap is essential as far as damping behavior of the chain tensioner is concerned. When dimensioning the leakage gap for a great leakage rate, the chain tensioner acts softer to dampen, e.g., chain knocks, as the piston is deflected to a greater degree. On the other hand, when the leakage rate is small, such chain knocks are damped under fairly small deflections of the piston, whereby greater forces are effective as a consequence of the smaller deflection. Therefore, a hard damping action of the chain tensioner is effected, when the leakage rate is only small.
Damping behavior of the chain tensioner is normally suited in optimum manner to an operating point. A soft damping action may be desired to avoid loud noises generated by the chain drive in the low speed range. The motor exhibits, however, a different operating behavior in the higher speed range, as more motor oil issues out of the pressure chamber through the leakage gap. Since hard chain knocks cannot be precluded, the piston is pushed further inwardly, when dimensioned for soft damping action, so that intake of motor oil into the pressure chamber will be insufficient. As a result, faulty operation of the chain tensioner can be experienced, in particular at higher speeds.
It would therefore be desirable and advantageous to provide an improved chain tensioner to obviate prior art shortcomings and to ensure a reliable operation.